This invention relates to a low alloy steel shape of at least 3/16 inch (4.8 mm) thickness having improved yield strength, toughness and excellent weldability, and to a novel process for the production thereof.
The steel shape of the invention may be produced, from a casting or previously rolled slab as a starting material, in the form of plate, bar, tube and structural shape, as hot reduced, hot reduced and precipitation hardened, or hot reduced, austenitized, quenched and precipitation hardened products. Although not so limited, the invention has particular utility in the production of plate of at least 3/16 inch thickness which retains good toughness in the heat affected zone of weldments made by any of the usual welding processes.
Steel plate hot rolled in accordance with the process of the invention exhibits a yield strength of at least 80 ksi (56 kg/mm.sup.2) at room temperature and a Charpy V-notch impact strength of at least 20 ft-lbs (27 Joules) in the longitudinal direction at -50.degree. F. (-46.degree. C.). When hot rolled in accordance with the process of the invention and precipitation hardened, steel plate exhibits a yield strength of at least 85 ksi (60 kg/mm.sup.2)at room temperature and a Charpy V-notch impact strength of at least 20 ft-lbs (27 Joules) longitudinal at -50.degree. F. (-46.degree. C.). When hot rolled in conventional manner, austenitized, quenched and precipitation hardened, the steel plate of the invention has a yield strength of at least 80 ksi (56 kg/mm.sup.2) at room temperature and a Charpy V-notch impact strength of at least 50 ft-lbs (68 Joules) longitudinal at -80.degree. F. (-62.degree. C.).
British Pat. No. 1,436,846, published May 26, 1976, discloses a steel alleged to exhibit good weldability and high strength containing from 0.4 to 0.8% nickel, 0.7 to 1.1% copper, 0.01 to 0.09% carbon, 0.02 to 0.1% niobium, 1.1 to 1.65% manganese, 0 to 0.5% chromium, 0 to 0.6% silicon, 0 to 0.5% molybdenum, 0 to 0.01% boron, 0 to 0.08% aluminum, 0 to 0.1% total of at least one of zirconium, magnesium, calcium, and rare earth metals, and balance iron except for impurities. Exemplary steels are stated to have a yield strength of at least 450 MN/m.sup.2 (65 ksi) and a toughness level such that the 70 J transition temperature is below -10.degree. C. when the hot rolled plate is finished at a temperature up to 1050.degree. C. Hot rolling finishing temperatures ranged from 950 .degree. to 1050.degree. C. in the specific examples.
ASTM A710 Grade B steel is based on U.S. Pat. No. 3,132,025, issued to Hurley May 5, 1964, which discloses a low alloy structural steel alleged to exhibit in the hot rolled condition an excellent combination of metallurgical properties. The steel contains up to 0.08% carbon, about 0.2 to 0.75% manganese, up to 0.35% silicon, about 1 to 1.7% copper, about 0.7 to 1.6% nickel, about 0.01 to 0.16% columbium (niobium) and balance essentially iron. Up to 0.1% aluminum may also be present. The steel is stated to exhibit a yield strength of at least 70 ksi at room temperature, a Charpy V-notch impact strength of at least 15 ft-lbs at -50.degree. F. and a reduction of area of at least 60%. The steel is processed by heating above about 2000.degree. F., hot rolled and finished at about 1650.degree. F. A precipitation-hardening treatment can also be applied by heating between about 850.degree. and 1150.degree. F. for about 1 hr.
U.S. Pat. No. 3,945,858, issued Mar. 23, 1976 to Matsubara et al, discloses a low alloy steel alleged to have high notch toughness at low temperature, comprising 0.02 to 0.10% carbon, 1.20 to 1.80% manganese, less than 0.015% sulfur, 0.05 to 0.50% chromium, 0.01 to 0.10% niobium, 0.10 to 0.50% silicon, less than 0.030% phosphorus, 0.05 to 0.50% nickel, 0.05 to 0.50% copper and balance iron and unavoidable impurities. A rare earth metal or alloy may be added within the range of 0.01 to 0.20%. The steel is hot rolled with a reduction in thickness of from 30 to 80% at a temperature below 950.degree. C. (1742.degree. F.). This is alleged to result in improved notch toughness.
U.S. Pat. No. 3,955,971, issued May 11, 1976 to Reisdorf, discloses a low alloy structural steel having good low temperature properties, such as a minimum yield strength of at least 60 ksi and good impact toughness down to temperatures as low as -80.degree. F. The steel comprises 0.06 to 0.12% carbon, 0.20 to 1.00% manganese, 0.020% maximum phosphorus, 0.015% maximum sulfur, 0.15 to 0.40% silicon, 0.75 to 1.50% nickel, 0.50 to 1.25% chromium, 0.15 to 0.40% molybdenum, 0.010 to 0.060% aluminum, 0.75% maximum copper with copper plus chromium being 1.50% maximum, and balance iron and conventional impurities. In the processing of exemplary 1 and 2 inch thick plates, samples were austenitized at 1650.degree. F., water quenched, and then tempered at 1150.degree., 1200.degree. and 1250.degree. F.
Other prior art or which applicant is aware includes U.S. Pat. Nos. 3,692,514, 3,841,866; 3,849,209 and 4,008,103, and Canadian Pat. No. 988,751.
ASTM alloy A710, Grade A, has an analysis, in weight percent, of 0.07% maximum carbon, 0.40 to 0.70% manganese, 0.025% maximum phosphorus, 0.025% maximum sulfur, 0.40% maximum silicon, 0.70 to 1.00% nickel, 0.60 to 0.90% chromium, 0.15 to 0.25% molybdenum, 1.00 to 1.30% copper, 0.02% minimum columbium, and balance iron. This composition is based on the above-mentioned U.S. Pat. No. 3,692,514.
Despite the numerous prior art developments in the field of low alloy steel plate for structural purposes, there is still a need for such a steel in plate form, particularly in thicknesses of 3/16 to 2" (4.8 to 51 mm), which consistently exhibits a yield strength of at least 80 ksi at room temperature, a Charpy V-notch impact strength of at least 20 ft-lbs (longitudinal) and at least 15 ft-lbs (transverse) at -50.degree. F. in the hot rolled condition; a yield strength of at least 85 ksi and a Charpy V-notch impact strength of at least 20 ft-lbs (longitudinal) and at least 15 ft-lbs (transverse) at -50.degree. F. in the rolled and precipitation hardened condition; and a yield strength of at least 80 ksi and a Charpy V-notch impact strength of at least 50 ft-lbs (longitudinal) and at least 35 ft-lbs (47 Joules) (transverse) at -80.degree. F. in the quenched and precipitation hardened condition, together with excellent weldability, particularly retention of toughness in the heat affected zone of a weldment made by any of the usual welding processes.